<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        body {
            border: 1px solid #000;
        }

        .case2 {
            width: 200px;
            height: 50px;
            background: red;
        }

        .case2-inner {
            margin-top: 50px;
            height: 0;
        }

        .float {
            float: left;
        }
    </style>
</head>

<body>
    <div class="case2">
        <div class="float"></div>
        <div class="case2-inner">看出了啥</div>
    </div>
</body>

</html>